1. Field of the Invention
The present invention relates to an electronic component having a capacitor element.
2. Description of the Related Art
Various types of surface-mount electronic components are mounted on a circuit inside an electronic apparatus such as a personal computer or portable telephone. Known surface-mount electronic components include thin-film type electronic components formed using thin film forming techniques.
Thin-film electronic components include thin-film capacitors, thin-film inductors, thin-film LC composite components, and thin-film multi-layer components. Composite components having a capacitor include low-pass filters (LPFs), high-pass filters (HPFs), band-pass filters (BPFs), and trap filters which eliminate signals in a predetermined frequency range. Further, those components may be combined to provide electronic components such as diplexers, duplexers, antenna switch modules, and RF modules.
There is demand for compactness and reductions in the height and cost of electronic components to be used at frequencies as high as 500 MHz or more and, more particularly, at frequencies in a micro-wave frequency band (GHz band). In the case of capacitors to be used at high frequencies, compactness and greater capacities have been pursued by employing a dielectric film made of a material having a high dielectric constant or reducing the thickness of a dielectric film. Further efforts toward capacitors having greater capacities include the use of multi-layer dielectric films and increasing the area of capacitor electrodes.
However, the use of a dielectric film made of a material having a great dielectric constant results in an increase in a dielectric loss tangent, which consequently results in an increase in transmission loss of a capacitor in the operational frequency range thereof. Under the circumstance, a dielectric film made of a material resulting in a small dielectric loss tangent is used for a capacitor to be used at a high frequency.
FIGS. 20A and 20B show a schematic configuration of a thin-film type capacitor element 411 according to the related art. FIG. 20A is a plan view of the capacitor element 411, and FIG. 20B is a sectional view of the same taken along the line A-A in FIG. 20A. As shown in FIGS. 20A and 20B, the capacitor element 411 includes a bottom conductor 421 formed on a substrate 51, a dielectric film 431 formed on the bottom conductor 421, and a top conductor 423 formed on the dielectric film 431. Part of the bottom conductor 421 and the top conductor 423 serves as an electrode of the capacitor element 411. An area l1×l2 which is the area of the dielectric film sandwiched between the bottom conductor 421 and the top conductor 423 is defined as the area of the electrode that is one factor determining the capacity value of the capacitor element 411.
In the capacitor element 411 according to the related art, the dielectric film 431 tends to be smaller in thickness at edges of the bottom conductor 421 than on a top surface thereof. When the thickness of the dielectric film 431 is reduced, the dielectric film 431 may not be formed on the bottom conductor 421. In this case, sufficient insulation may not be provided between the bottom conductor 421 and the top conductor 423 at the edges of the bottom conductor 421, which increases the possibility of a shorting failure. As a result, a breakdown limit of the withstand voltage of the capacitor element 411 may be lowered, and a problem therefore arises in that the quality of products becomes inconsistent in terms of voltage withstanding capability. A shorting failure or a reduction in the breakdown limit of the withstand voltage is likely to occur when the thickness of the dielectric film 431 is small relative to the thickness of the bottom conductor 421 or top conductor 423 or when the edges of the bottom conductor 421 are inversely tapered.
Under the circumstance, the dielectric film 431 is formed from a material having high insulating properties or formed with a great thickness in an intention to improve the withstand voltage of the capacitor element 411. However, an increase in the thickness of the dielectric film 431 necessitates an increase in the electrode area of the capacitor element 411 in order to obtain a great capacity, which results in a problem in that it becomes difficult to make the electronic component compact. Further, although the dielectric film 431 covers the top surface and edges of the bottom conductor 421, the thickness of the dielectric film 431 at the edges is smaller than the thickness of the dielectric film 431 on the top surface of the bottom conductor 421 in most cases. When the thickness of the bottom conductor 421 is set at a great value or when the bottom conductor 421 is provided with a great wiring length taking equivalent series resistance (ESR) or parasitic impedance into consideration, a capacity formed between the edges of the bottom conductor 421 and the top conductor 423 has a great capacity value. Thus, variation of the thickness of the dielectric film 431 covering the edges of the bottom conductor 421 has an adverse effect on the achievement of a desired capacity value.
Referring to thin-film type electronic components having a capacitor, reductions in the electrode area and the number of dielectric films have a great importance in providing a small-sized and low-profile capacitor operating at a high frequency at a low cost. The accuracy of the capacity value of the capacitor element 411 depends on the relative positional accuracy of the bottom conductor 421 and the top conductor 423, the accuracy of the shape of the bottom conductor 421 or the top conductor 423, the accuracy of the thickness and dielectric constant of the dielectric film 431, and the surface roughness of the bottom conductor 421 and the top conductor 423. In the case of the capacitor element 411, a general way to improve the relative positional accuracy of the top conductor 423 and the bottom conductor 421 is to change the electrode area of each conductor.
The top conductor formed above the bottom conductor must be smaller in dimensions than the bottom conductor in consideration to a possible shorting between the top conductor and the bottom conductor and their positional accuracy. In particular, when conductors having small dimensions are used, there is a limit on the number of top conductors that can be formed because top conductors are formed in small dimensions to allow some margin for the positional accuracy thereof. Further, the use of a multi-layer top conductor does not necessarily result in an increase in the capacity of a capacitor. In capacitors according to the related art, since a dielectric film is formed in low compliance with a bottom conductor at edges of the conductor, a top conductor is formed so as to avoid the edges in order to prevent shorting between the top conductor and the bottom conductor. Further, bottom conductors of capacitors according to the related art are uneven in the shape of edges thereof. When such edges are used as part of electrodes of the capacitors, there will be significant variations in the electrode area, and the capacity value cannot be accurately controlled. Under the circumstance, in capacitors according to the related art, a dielectric film and a top conductor are formed in the order listed on a planar part of a bottom conductor excluding edges thereof, and a capacity is formed by the area (electrode area) in which the top conductor and the bottom conductor face each other and the thickness of the dielectric film.
In the electronic component having a capacitor, a parasitic inductance or floating capacity is suppressed by adjusting the circuit layout to reduce the distance from the conductors of the capacitor element 411 to terminals and to reduce the length of a lead-out conductor for connecting the capacitor element 411 and a circuit element adjacent to the capacitor element 411.
However, since part of the lead-out conductor is in contact with the dielectric film 431, the capacity value of the capacitor element 411 is different from a design value when there is any misalignment between the positions where the bottom conductor 421 and the top conductor 423 are formed. For example, the lead-out conductor is formed with a small width in order to suppress the deviation of the capacity value of the capacitor from the design value. However, since a reduction in the width of the lead-out conductor results in an increase in the parasitic inductance, there will be problems including degradation of high-frequency characteristics of the electronic component and an increase in transmission loss.
In the thin-film type capacitor element disclosed in Patent Document 1, a bottom electrode and a dielectric layer are formed in the order listed on a substrate, and the periphery of the dielectric layer is covered by an insulator layer having an opening. A top electrode formed on the insulator layer overlies on the dielectric layer in the opening. In such a configuration, the insulator layer covering the periphery of the dielectric layer provides reliable insulation between the bottom electrode and the top electrode. As a result, any reduction or variation of a breakdown voltage attributable to insufficient coverage of the dielectric layer can be reliably prevented. Further, since the capacity value of the capacitor is determined by the opening of the insulator layer, variation of the capacity value can be reduced regardless of the size of the bottom and the top electrodes and the accuracy of alignment of the electrodes.
However, in the capacitor element disclosed in Patent Document 1, since the top electrode is formed also in the same layer as the bottom electrode so as to face the bottom electrode with the insulator layer interposed between them, a parasitic capacity is generated between the electrodes. Since the insulator layer is formed to protrude above the substrate surface, it is difficult to provide the dielectric layer in a multi-layer structure. Further, the configuration makes it difficult to provide a composite component having a plurality of circuit elements in a small size because circuit elements such as an inductor element cannot be formed close to the capacitor element.
Patent Document 1: JP-A-2002-25854
Patent Document 2: JP-A-2002-33559
Patent Document 3: JP-A-2003-17366
Patent Document 4: Japanese Patent No. 3193973